BACKGROUND ART
It is known that a latent electrostatic image can be developed with toner particles dispersed in an insulating nonpolar liquid. Such dispersed materials are known as liquid toners or liquid developers. A latent electrostatic image may be produced by providing a photoconductive layer with a uniform electrostatic charge and subsequently discharging the electrostatic charge by exposing it to a modulated beam of radiant energy. Other methods are known for forming latent electrostatic images. For example, one method is providing a carrier with a dielectric surface and transferring a preformed electrostatic charge to the surface. Useful liquid toners comprise a thermoplastic resin and nonpolar liquid. Generally a suitable colorant is present such as a dye or pigment. The colored toner particles are dispersed in the nonpolar liquid which generally has a high-volume resistivity in excess of 10.sup.9 ohm centimeters a low dielectric constant below 3.0 and a high vapor pressure. The toner particles are less than 10 .mu.m average by area size as measured hy a Horiba CAPA-500 centrifugal automatic particle analyzer. After the latent electrostatic image has been formed, the image is developed by the colored toner particles dispersed in said nonpolar liquid and the image may subsequently be transferred to a carrier sheet.
Many methods for making liquid electrostatic developers are known. For example, as described in Landa et al. U.K. patent application No. 2169416A published July 9, 1986, electrostatic toners are prepared by plasticizing a thermoplastic polymer and pigment with a nonpolar liquid to form a gel or solid mass and shredding the mass into pieces. After the addition of more nonpolar liquid, the pieces are ground into toner particles.
Another process for the preparation of liquid electrostatic developer is described in Larson U.S. Ser. No. 804,385 filed Dec. 4 1985. This process comprises (a) dispersing in a nonpolar liquid a thermoplastic resin and optionally a colorant using moving particulate media, the temperature being sufficient to plasticize and liquefy the resin and below the decomposition temperature of the resin and/or colorant and below the nonpolar liquid boiling point; (b) cooling the dispersion while maintaining continuous movement of the particulate media during and subsequent to cooling whereby electrostatic toners particles are formed.
These and other processes for preparation of liquid electrostatic developers are disadvantageous because they require relatively long preparation time primarily due to long cold grinding cycles for particle size reduction.
It is therefore desirable that liquid electrostatic developers having properties equal or better than those prepared by previous methods be prepared by a new method in a shortened time period.